The problem, when the liquid of a bladder tank or tank built into a structure, such as for example an aircraft or a land-based vehicle, is passed through by a projectile, is that of the overpressure that is generated. This overpressure within the liquid may lead to the destruction of the tank by rupture.
Indeed, the projectile displaces, over its trajectory, a volume of liquid which, by nature, is generally incompressible.
In current tanks, there is no possible expansion of the volume containing the liquid, when a projectile passes through the liquid, which means that a pressure is applied increasingly to the walls of the tank until the walls of the tank rupture. More specifically, the interaction between the projectile and the fluid produces, for the first one, a loss of kinetic energy and, for the second one, an increasing displacement which, if it is not compensated for, will cause this pressure rise. The resulting pressure wave will then strike the wall of the fuel tank over a relatively large zone and, as a function of the amplitude of this wave and the construction of the fuel tank, may break the wall of the tank.
Currently, no tank makes it possible to completely manage this bulk pressure. The risk of losing an aircraft or vehicle by explosion of the tank after a ballistic impact is therefore very high.
Certain bladder tanks, in particular those equipped with a double wall surrounding a foam with a self-sealing function after perforation, for example in the case of certain military aircraft, may optionally limit the effects of gunfire. However, such a foam is far from being suitable for absorbing the overpressure generated by the penetration of the projectile.
Various known techniques may respond more or less to the objective of reducing the vulnerability of a tank faced with a ballistic threat. These techniques that aim to reduce the effects of overpressure are the following:                nesting of two tanks (also referred to as a “tank in tank” system), the one inside the largest one having to break under the ballistic impact and empty its contents into the other,        use of a rubber coating inside the tank (also referred to as “rubber layers around tank”), making it possible to separate the fluid from the structure that forms the tank,        use of a fiber-reinforced rubber coating inside the tank, which is a derivative of the preceding concept,        use of a Kevlar bladder tank filled with open-cell foam (also referred to as “foam in bladder tank”),        use of a bladder tank with inflatable walls (also referred to as a “nitrogen inflated ballistic bladder” system), described in U.S. Pat. No. 4,925,057,        use of corrugated reinforcements made of glass fibers/resin bonded between the rigid-walled tank and the structure containing the latter (taking up a principle described in U.S. Pat. No. 4,469,295 and U.S. Pat. No. 7,566,489) and that can be deformed under the effect of overpressure,        use of a sacrificial layer lining the inside of the tank, made of honeycomb blocks covered with a skin made of glass fibers/resin (for sealing the cells), that can be crushed under the effect of overpressure.        
However, these tanks are not suitable for high pressures and furthermore do not make it possible to limit the damage caused by several successive or simultaneous projectile impacts.
Also known from patent EP 2048079 is a fuel tank assembly and a corresponding method that makes it possible to limit the damage that the impact of a ballistic projectile could cause, in particular hole-type damage that promotes a leakage of fuel. This patent describes a double tank formed by two nested bladders and that is thus capable of limiting the damage caused by a ballistic impact. The assembly also comprises a plurality of connectors positioned between the two bladders in order to at least partially limit the expansion of the bladder. Thus, this assembly makes it possible to limit the expansion of the internal bladder, but does not make it possible to prevent the explosion of the tank if the pressure exerted by the volume expansion of the fuel during the penetration of the projectile is too high.
Lastly, a product based on polyurethane foam is known that protects fuel tanks solely against electrostatic effects and sloshing, while providing the inerting of the vapors and a barrier against external debris. This system does not make it possible to manage the overpressures generated by a ballistic impact.